1. Field of the Invention
The present invention relates to a semiconductor optical device including a semiconductor optical element which emits light, and to a semiconductor laser module using the semiconductor optical device.
2. Description of the Background Art
A structure of a conventional semiconductor optical device, which is disclosed in Japanese Patent Laying-Open No. 2001-196685, will now be described with reference to FIG. 19. As shown in FIG. 19, the conventional semiconductor optical device includes a semiconductor laser 101, which is an example of the semiconductor optical element and which has an effective refractive index nc, and a nonreflective film provided in contact with a light emission surface of semiconductor laser 101.
In addition, the nonreflective film includes a first film 102 provided in contact with an end surface of semiconductor laser 101 and having a refractive index n1 and a thickness d1, a second film 103 provided in contact with an end surface of first film 102 and having a refractive index n2 and a thickness d2, and a third film 104 provided in contact with an end surface of second film 103 and having a refractive index n3 and a thickness d3.
FIG. 20 shows wavelength dependence data of a reflectance of a nonreflective film used in a semiconductor laser having an effective refractive index nc=3.2. The data shown in FIG. 20 shows wavelength dependence of the reflectance of the nonreflective film when the semiconductor laser emits light having a wavelength λ=1.3μm.
Data 105 in FIG. 20 shows wavelength dependence of the reflectance of a nonreflective film of alumina having refractive index n1=1.6 and thickness d1=106.2 nm, amorphous silicon having refractive index n2=3.2 and thickness d2=10.6 nm, and silicon oxide having refractive index n3=1.45 and thickness d3=73.9 nm.
Data 106 in FIG. 20 shows wavelength dependence of the reflectance of a nonreflective film of alumina having refractive index n1=1.6 and thickness d1=512.5 nm, amorphous silicon having refractive index n2=3.2 and thickness d2=10.6 nm, and silicon oxide having refractive index n3=1.45 and thickness d3=73.9 nm.
When the effective refractive index of semiconductor laser 101 is nc=3.2, the refractive index, which is the square root of the effective refractive index, is nf=1.78885. In addition, when the light has wavelength λ=1.3 μm, one quarter wavelength λ is 325 nm.
As shown in data 105 of the wavelength dependence, when the total film thickness of the above-mentioned three kinds of films (d1×n1+d2×n2+d3×n3) is nearly equal to a quarter of λ, a range of a wavelength (referred to as a “low-reflectance wavelength range” hereinafter), wherein the nonreflective film functions with a low reflectance equal to or lower than 1%, is 257 nm.
On the other hand, as shown in data 106 of the wavelength dependence, when the total film thickness of the three films (d1×n1+d2×n2+d3×n3) is changed to about 961 nm in order to enhance the thermal conductivity of the nonreflective film, the low-reflectance wavelength range is 78 nm.
In the conventional semiconductor optical device, the above-mentioned three kinds of films are used as the nonreflective film of the semiconductor laser as an example of the semiconductor optical element. When the total thickness of the three films (the total sum of products of thicknesses and refractive indices of respective layers) is an integer multiple of λ/4 other than λ/4, however, the low-reflectance wavelength range becomes extremely narrow. More specifically, it is difficult to make the low-reflectance wavelength range equal to or wider than 100 nm. Therefore, in the conventional semiconductor optical device disclosed in Japanese Patent Laying-Open No. 2001-196685, the total film thickness must be equal to λ/4.
Therefore, as means to solve the above-described problem caused with the conventional semiconductor optical device disclosed in Japanese Patent Laying-Open No. 2001-196685, the inventors of the present invention have been studying forming a nonreflective film by a design procedure in which a real part and an imaginary part of an amplitude reflectance of two films having different compositions are respectively brought to zero, as an unpublished technique as of the application for patenting of the present invention. With this design procedure, the total film thickness of two films having different compositions can be made different from λ/4.
In this design procedure, however, the degree of freedom in a design of a nonreflective film of a semiconductor optical device cannot be enhanced because only two kinds of films form the nonreflective film. With this design procedure, for example, it is impossible to provide a film for efficient heat radiation by a semiconductor laser as a third film in addition to the two films respectively bringing the real part and imaginary part of the amplitude reflectance to zero.